Method of longitudinally sealing tubular bag-making material

ABSTRACT

A packaging machine moves its heater unit for longitudinal sealing of a tubularly formed film between a sealing position at which the heater unit contacts the film and a retracted position at which it is separated from the film. In addition to the mechanism for this motion, an air cylinder is provided for controlling the compressive force with which the heater unit at its sealing position compresses the film. A pressure controlling system regulates the air pressure supplied to the air cylinder. If the mechanism for moving the heater unit also employs another air cylinder, a higher pressure for moving the heater unit and a lower pressure for controlling its compressive force may be generated from a single air source and supplied to the two air cylinders by a switching mechanism. A single air cylinder with two air chambers may be used for the both functions. A motor is additionally provided for removing the heater unit and its support unit much farther away from the film to provide a large enough space for exchanging the former and/or the chute of the packaging machine.

BACKGROUND OF THE INVENTION

This invention relates to a method of using a form-fill-seal packagingmachine for forming bags while concurrently filling and sealing themwith articles to be packaged and more particularly to a method oflongitudinally sealing a tubularly formed bag-making material to form abag by using such a bag-making material.

In general, a packaging machine of this type is structured so as to bendan elongated web of thermoplastic bag-making material (herein referredto as the “film”) into a tubular form around a chute through whicharticles are dropped to be packaged, to longitudinally seal its mutuallyoverlapping side edges by means of a longitudinal sealer and then toseal the tubularly formed film transversely to produce a packagedproduct.

The aforementioned process of longitudinal sealing is usually effectedby causing a heated heater block to intermittently come into contactwith the chute so as to sandwich and compress the overlapped edge partsof the film between the block and the outer front surface of the chutewhile the film is being transported longitudinally (the intermittentmode of film transportation) or by leaving a heated metallic heater beltin contact with the chute such that the overlapped edge parts of thefilm is continuously pressed against each other between the block andthe chute (the continuous mode of film transportation). In what follows,such a heater block and a heater belt will be both referred to as the“heater” and discussed together for convenience.

In either mode of transportation described above, the heater is adaptedto move between a sealing position where the heater comes into contactwith the chute to effect the longitudinal sealing and a retractedposition to which the heater is moved away from the chute. In theintermittent mode of transportation, the heater is moved to a retractedposition while the film is being transported. In the continuous mode oftransportation, on the other hand, the heater is moved to the retractedposition when, for example, a trouble has occurred and thetransportation of the film is interrupted temporarily in order toprevent the overheating of the film.

While the heater is at the sealing position, it presses the film againstthe chute with a specified pressure for the thermal sealing. Thiscompressive pressure is usually supplied by the biasing force of aspring, and it has been the practice to adjust its compressive force. Itsometimes happens when the chute is attached to the main frame of thepackaging machine, however, that the chute is not exactly at theintended position, depending on the manner in which the attachment iseffected, or that the relative position changes or the shape of theheater changes after many years of use. In such a situation, the sealingposition of the heater may change and the compressive force by thespring may also change. Thus, the compressive force of the spring had tobe adjusted as the sealing position of the heater changed.

Manual adjustment of the spring could not be done very accurately.Moreover, the recent trend is that the compressive force of the springmust be adjusted not only according to the film characteristics such asits thickness and material but also according to the method of sealingsuch as the so-called lap sealing and fin sealing and also according tothe operational capabilities and conditions. In other words, fineadjustments of the compressive force of the spring according to manyfactors are now a commonly required procedure.

As explained above, furthermore, the heater must generally be movedbetween a sealing position and a retracted position. A quick response isrequired in this motion such that the heater can move between thesepositions as instantaneously as possible. This means that a fairly largeforce is necessary for the motion of the heater and further that adifferent kind of improved control is necessary. Thus, there is now anew problem of how to control these two kinds of forces on the singleheater.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a packagingmachine capable of efficiently effecting both the control on the motionof the heater between its sealing and retracted positions and thecontrol of the compressive force of the heater at is sealing position.

A packaging machine embodying this invention, with which the above andother objects can be accomplished, may be characterized as comprisingnot only a heater driving means for moving a heater unit forlongitudinal sealing of a tubularly formed film between a sealingposition at which the heater unit contacts the film and a retractedposition at which it is separated therefrom, but also an air cylinderfor controlling the compressive force with which the heater unit at thesealing position compresses the film against a cylindrical chute aroundwhich the film is formed into a tubular form. Means for regulating theair pressure supplied to this air cylinder is provided, and a controllerserves to control and vary this specified level. The air cylinder forcontrolling the pressure actually does not operate to change therelative position of the heater unit with respect to the chute. Theheater unit is supported by a support unit which is slidable towards oraway from the chute and the heater driving means serves to operate onlyon the support unit, while the air cylinder operates only on the heaterunit, not on the support unit, although this heater driving means mayitself comprise another air cylinder. A separate motor may also beprovided for removing the heater unit and its support unit much fartheraway from the chute, say, for exchanging the former and/or the chute ofthe packaging machine.

With a packaging machine thus characterized, the force with which thefilm is compressed by the heater unit to effect longitudinal sealing canbe accurately controlled, say, according to the characteristics of thefilm such as its material and its thickness, because an air cylinder forthis particular purpose is provided apart from the mechanism for movingthe heater unit between its sealing and retracted positions. Even if thesealing position by the heater unit may have changed due to deformationof the chute after many years' use, for example, it is possible to applythe same compressive force on the film. Moreover, fine adjustments ofthe compressive force becomes possible according to this invention. If asecond air cylinder is used for moving the heater unit between itssealing and retracted positions, furthermore, the responsecharacteristics of the heater unit can be improved by using the samesource of air pressure used for the air cylinder for the control of thecompressive force.

The moving of the heater unit between its sealing and retracted positionand the control of its compressive force on the film for effecting itslongitudinal sealing may be carried out by using a single air cylinderhaving two chambers. Such an air cylinder will be provided with anair-pressure generating means for generating from a single air sourceboth a higher air pressure for moving the heater unit and a lower airpressure for controlling the compressive force of the heater unit. Aswitching means will function such that the higher and lower pressuresthus generated will be supplied to appropriate ones of these chambers ofthe air cylinder selectively, depending on whether the heater unit isbeing moved towards the sealing position or the retracted position, orits compressive force is being adjusted. A controller may be furtherprovided such that the higher pressure can be applied to the aircylinder for a specified length of time while the lower pressure isbeing supplied.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a schematic front view of a packaging machine embodying thisinvention;

FIG. 2 is a plan view of a center portion of the packaging machine ofFIG. 1;

FIG. 3 is an enlarged side view of the heater unit of the packagingmachine of FIG. 1;

FIG. 4 is a sectional view taken along line 4—4 of FIG. 2;

FIG. 5 is a sectional view taken along line 5—5 of FIG. 2;

FIG. 6 is a sectional view taken along line 6—6 of FIG. 2;

FIG. 7 is a plan view of the rotary frame of the packaging machine ofFIG. 1, showing its rotary motion;

FIG. 8 is a block diagram of the control system of the packaging machineof FIG. 1 for the longitudinal sealing;

FIG. 9 is a plan view of the center portion of the packaging machine ofFIG. 1 when the heater unit is compressed against the chute;

FIG. 10 is a plan view of a center portion of another packaging machineembodying this invention;

FIG. 11 is a plan view of a center portion of still another packagingmachine embodying this invention; and

FIG. 12 is a block diagram of the control system of the packagingmachine of FIG. 11 for the longitudinal sealing.

Throughout herein, like or equivalent components are indicated by thesame numerals and may not necessarily be described repetitiously forsimplifying the disclosure even where these components are components ofdifferent packaging machines.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described next by way of an example with reference toFIG. 1 wherein numeral 1 generally indicates a packaging machine 1embodying this invention. Although not shown, there is a film holder forrotatably supporting a roll of elongated film at a backward positionabove its frame 2. On the front side of the machine frame 2 at elevatedpositions are a former 3 for bending the film pulled out of the roll ina forward direction into a tubular form while the film is being guidedalong a downward film path such that the mutually opposite side edges ofthe film become overlapped, a longitudinal sealer 40 for sealingtogether these mutually overlapped side edges of the film to make itinto a tubular form, and a cylindrical chute 4 through which articles tobe packaged are dropped into such a tubularly formed film. Numeral 5indicates a support frame attached to the machine frame 2 for supportingthe former 3 and the chute 4.

A pair of film transporting mechanisms 10 is provided on both(right-hand and left-hand) sides of the chute 4 and at about a half ofits height. Although not shown in detail, each of the film transportingmechanisms 10 includes a pair of upper and lower rollers 12 supported bya covering member 11 attached to the machine frame 2. A belt member 13is stretched over and between the rollers 12 such that the tubularlyformed film is sandwiched from both sides between the pair of beltmembers 13 and compressed against the right-hand and left-hand sidesurfaces of the chute 4. Although not shown, a belt-rotating motor isalso provided for rotating the rollers 12 such that the tubularly formedfilm is transported downward longitudinally along the chute 4.

Inside the machine frame 2 is a transverse sealer 20 having a pair ofbar members 21 (only one shown in FIG. 1) disposed in front of andbehind the tubularly formed film and support units 22 which support thebar members 21 such that these bar members 21 can move towards or awayfrom each other. These bar members 21 include a heater (not shown) andare heated thereby. As these bar members 21 are moved towards eachother, they sandwich the film in between and seal it transversely to itslongitudinal direction of motion along two parallel seal lines. At thesame time, the film is cut by a cutter attached to one of the barmembers 21 along a line between the two seal lines. Numeral 30 indicatesa table disposed at the bottom of the machine frame 2 for depositing thepackaged products produced by this packaging machine 1.

Next, the longitudinal sealer 40 will be described more in detail withreference to FIGS. 2 and 3. The longitudinal sealer 40 is attached to arotary frame 41 supported rotatably by a main frame 6 and comprises aheater unit 50 for sealing the mutually overlapped side edges of thefilm, a support unit 60 which serves to support the heater unit 50 andis slidable with respect to the rotary frame 41 and a moving mechanism70 for moving the heater unit 50 towards or away from the chute 4. Theheater unit 50 includes, as shown in FIGS. 2 and 3, a pair of upper andlower pulleys 52 attached to an attachment plate 51, a heater belt 53stretched over and between these pulleys 52, a heater block 54 which isdisposed between the pulleys 52 and serves to heat the heater belt 53,and a belt-moving motor 55 for running the heater belt 53. The heaterunit 50 operates the heater belt 53 so as to compress the film againstthe chute 4 while running in the direction of travel of the film and tothereby continuously seal the mutually overlapping side edgeslongitudinally together. The support unit 60 comprises an arm member 61which extends sideways (in the right-left direction), a rail member 62attached to an inner side surface of the rotary frame 41 and a slidermember 63 which is connected to the arm member 61 and engages slidablywith the rail member 62 so as to be able to slide therealong in theforward-backward direction and to thereby cause the heater unit 50 tomove between the sealing position where the heater belt 53 contacts theoverlapped parts of the film to seal them together (as shown in FIG. 9)and the retracted position (as shown in FIG. 2) where the heater belt 53is no longer in contact with the film. Stoppers 63 a are provided atboth end positions of the rail member 62 so as to limit the slidingmotion of the slider member 63.

Next, the manner in which the heater unit 50 is supported by the supportunit 60 will be explained more in detail. On the surface of theattachment plate 51 of the heater unit 50 opposite to the surface onwhich the pulleys 52 are attached, as shown in FIGS. 2 and 4, there is asupport plate 56 which is connected to the arm member 61 by means of apair of mutually parallel link mechanisms 64 (of which only link members64 a and 64 b are seen). An air cylinder 66 (herein referred to as the“first air cylinder”) is attached to the frontal edge surface 61 a ofthe arm member 61 through an attachment plate 65. One of the linkmembers (64 a) closer to the chute 4 has its center part extendeddownward and the tip of the piston rod 66 a of the air cylinder 66 isattached to this extended portion 64 a′ of the link member 64 a. Thus,the compressive force of the heater belt 53, when the heater belt 53comes into contact with the overlapped portions of the film at thesealing position, can be adjusted by means of this air cylinder 66.

The moving mechanism 70 is contained inside the rotary frame 41, asshown in FIG. 2, and comprises another air cylinder 71 (herein referredto as the “second air cylinder”) with a piston rod 71 a having its tipfastened to the slider member 63, an attachment plate 72 for fasteningthis air cylinder 71, a motor 73 attached to an inner surface of therotary frame 41 and a screw bar 74 attached to the drive shaft (notshown) of this motor 73. As shown in FIG. 5, the interior of the secondair cylinder 71 is partitioned by means of a piston member 71 b into afirst air supply chamber 71 c and a second air supply chamber 71 d, andthere are also provided air passages 79 a and 79 b (shown in FIG. 8) forsupplying air from an external compressed air source (shown at 100 inFIG. 8) therethrough. The second air cylinder 71 is adapted to functionsuch that the piston rod 71 a will stick out to move the heater unit 50and its support unit 60 to the aforementioned retracted position if airis supplied into the first chamber 71 c through the air passage 79 a toraise its internal pressure and that the piston rod 71 a will move backto move the heater unit 50 and its support unit 60 to the sealingposition if air is supplied into the second chamber 71 d through the airpassage 79 b to raise its internal pressure.

The attachment plate 72 for the second air cylinder 71 engages with thescrew bar 74, and guide members 75 are provided on the lower surface ofthe rotary frame 41 such that the attachment plate 72 will be preventedfrom undergoing any rotary motion when the motor 73 is activated butwill move longitudinally along the screw bar 74 while remaining in theupright position. As explained above, the heater unit 50 is movedbetween the sealing position and the retracted position during a normalcourse operation. Thus, the distance between the sealing position andthe retracted position is relatively short. An area in which thesepositions are found will be herein referred to as the “work area”. Whenthe chute 4 is replaced by another with a different size or when theinitial position of the heater unit 50 or its support unit 60 isautomatically set according to the size or the set position of the chute4, for example, the heater unit 50 is removed much farther away from theworking area so as to provide a sufficiently large space, say, forexchanging the chute 4. Such an farther removed area will be hereinreferred to as the “non-work area”. The motor 73 may thus be describedas serving to move the heater unit 50 between the work and non-workareas. The attachment plate 72 has a protruded portion 72 a at itscenter where it is penetrated by the screw bar 74 so as to make iteasier for the attachment plate 72 to move along the screw bar 74. InFIGS. 4 and 7, numeral 6 a indicates a stopper for not allowing therotary frame 41 to rotate further in the clockwise direction (asindicated by arrow Y in FIG. 7) from the set position indicated bydotted lines, although it is allowed to move in the counter-clockwisedirection (as indicated by arrow X in FIG. 7), say, to reach theposition shown by solid line in FIG. 7.

Next, the air supply system for supplying air to the first and secondair cylinders 66 and 71 will be explained. As shown in FIG. 8, these aircylinders 66 and 71 are each provided with a regulator 101 or 102(herein referred to as the “first regulator” 101 and the “secondregulator” 102 for the convenience of description) for providingspecified air pressures. As briefly mentioned above, the packagingmachine 1 is provided with an air pressure source 100 for generating anair pressure for supplying air into the bags to be made. Each of theseregulators 101 and 102 is connected to the air pressure source 100 andis adapted to adjust the air pressure of the source 100 to a specifiedlevel. The air supply system further includes a switch valve 103 forselectively connecting the second regulator 102 to either of the airpassages 79 a and 79 b. If the switch valve 103 opens the first airpassage 79 a, a specified higher air pressure prepared by the secondregulator 102 is applied to the first supply chamber 71 c and the pistonrod 71 a protrudes from the second air cylinder 71 such that the supportunit 60 moves the heater unit 50 to the retracted position. If theswitch valve 103 opens the second air passage 79 b, the piston rod 71 ais retracted into the second air cylinder 71 such that the support unit60 moves the heater unit 50 to the sealing position.

The first regulator 101 is for supplying air having a specified lowerpressure to the first air cylinder 66. When the first regulator 101 isactivated, the piston rod 66 a of the first air cylinder 66 protrudes,causing the heater unit 50 to move towards the chute 4. The operationsof the switch valve 103 and the regulators 101 and 102 are controlled bya controller 105.

When the packaging machine 1 is started, the controller 105 activatesthe motor 73 to move the heater unit 50 and its support unit 60 to thepreliminarily determined sealing position as shown in FIG. 9 and alsothe first regulator 101 so as to adjust the air pressure and to therebypress the heater belt 53 against the chute 4 at the sealing position bymaintaining a constant compressive force. If a trouble occurs during thesealing process by the heater belt 53 such as the breaking of the film,the second air cylinder 71 is activated to move the heater unit 50 andits support unit 60 to the retracted position. At the time of exchangingthe chute 4 and the former 3, the motor 73 is operated to move theheater unit 50 not only to the retracted position but also to thenon-work area farther away from the chute 4 because a sufficient spacecannot be secured for exchanging the chute 4 if the heater unit 50 andits support unit 60 are moved only to the aforementioned retractedposition which is still within the work area.

In summary, the packaging machine 1 thus structured is characterized asusing two air cylinders 66 and 71 and two regulators 101 and 102 for thesealing operation by the heater belt 53. Unlike the prior art packaging,machine which used a spring to adjust the compressive force by which theheater belt carries out the sealing, therefore, there is no need for amanual adjustment of the compressive force of the spring when the chutehas been deformed after many years of use. Instead, a desiredcompressive force can be applied even after the chute has been deformedor the position of its attachment has been changed such that the sealingposition has undergone a change. Adjustments can be easily effectedaccording to this invention merely by moving the heater belt between acontacting position and a retracted position and the heater belt canalso be moved between the work and non-work areas by means of only asingle air supply system. Since the means for driving the heater unit 50and its support unit 60 and the means for adjusting the compressiveforce of the heater unit 50 are both comprised of an air cylinder,furthermore, one single air pressure source can control both of thesemeans, and this serves to simplify the structure of the packagingmachine. The controller 105 may be further adapted to preliminarilyregister a plurality of air pressure values such that automatic controlor remote control operations can be effected thereby.

FIG. 10 shows another packaging machine which may be regarded as avariation of the packaging machine described above with reference toFIG. 19. It is different only regarding the shape and the positioning ofits first air cylinder for adjusting the pressure with which the heaterunit is to compress the film for longitudinal sealing. Hence, the otherlike or equivalent components are indicated in FIG. 10 by the samenumerals as in FIGS. 1-9 for convenience and are not explainedrepetitiously.

FIG. 10 also shows that the support plate 56, attached to the attachmentplate 51 of the heater unit 50, is connected with the arm member 61′ bymeans of a pair of mutually parallel link mechanisms 64′ (of which onlylink members 64 c′ and 64 d′ are seen), but its pressure-controlling aircylinder 66′ is not disposed inside the link mechanisms 64′ but its backend is supported rotatably by a support member 61 a′ attached to the armmember 61′ and the tip of its rod 66 a′ is rotatably connected to asupport part 64 e′ provided on the outer surface of an extended portion64 d″ of the link member 64 d′ away from the chute 4. If the rod 66 a′is pushed out of the air cylinder 66′, the heater unit 50 is shiftedtowards the chute 4. If the rod 66 a′ is retracted into the air cylinder66′, the heater unit 50 is moved away from the chute 4. In this manner,the pressure to be exerted on the film by the heater unit 50 foreffecting longitudinal sealing can be controlled.

Next, FIGS. 11 and 12 are referenced to explain another packagingmachine according to a different embodiment of the invention. Since thisembodiment is different only in the manner in which its heater unit 50is supported by the arm member 61, FIG. 11 employs the same numerals toindicate like or equivalent components.

As shown in FIG. 11, the heater unit 50 according to this invention isnot connected to the arm member 61 by means of a movable linkingmechanism 64 but is affixed to the arm member 61 and the pressureexerted by the heater unit 50 onto the film for longitudinal sealing iscontrolled by the same air cylinder 71 which also serves to move theheater unit 50 between the aforementioned sealing position and retractedposition.

The air supply system for supplying air to the air cylinder 71 of thepackaging machine of FIG. 11 is explained next with reference to FIG. 12wherein like components as described above with reference to FIG. 8 areagain indicated by the same numerals.

As explained above and again shown in FIG. 12, the air cylinder 71 hastwo air supply chambers (the “first air supply chamber 71 c” and the“second air supply chamber” 71 d) and is connected to two regulators(herein the “first regulator 101” and the “second regulator 102”), bothadapted to receive air from a common air supply source 100 which alsoserves to supply air to be introduced into the bags which are formed bythe packaging machine. The first regulator 101 is for supplying air at ahigher pressure for moving the heater unit 50 and the second regular 102is for supplying air at a lower pressure for controlling the pressure tobe exerted by the heater unit 50 for effecting longitudinal sealing. Airpassages 78 a, 78 b, 78 c and 78 d, as well as two switch valves (the“first switch valve 103” and the “second switch valve 104”) are alsoconnected as shown. Air passages 78 a and 78 b are respectively forsupplying the higher-pressure air prepared by the first regulator 101through the first switch valve 103 to the first or second air supplychamber 71 c or 71 d of the air cylinder 71. Air passage 78 c is forsupplying the lower-pressure air prepared by the second regulator 102 tothe second air supply chamber 71 d of the air cylinder 71 through thesecond switch valve 104. If the first switching valve 103 opens the airpassage 78 a and closes the air passage 78 b and the second switchingvalve 104 closes the air passage 78 c, air with the higher pressure issupplied into the first air supply chamber 71 c such that the heaterunit 50 is moved to the retracted position. If the first switching valve103 closes the air passage 78 a and opens the air passage 78 b while theair passage 78 c is closed by the second switching valve 104, air withthe higher pressure is supplied into the second air supply chamber 71 dsuch that the heater unit 50 is moved from the retracted position to thesealing position. If the air passages 78 a and 78 b are both closed bythe first switching valve 103 while the air passage 78 c is opened bythe second switching valve 104, air with the lower pressure is suppliedinto the second air supply chamber 71 d such that the compressive forceof the heater belt 53 at the sealing position is controlled. Theseswitching valves 103 and 104 and the regulators 101 and 102, as well asthe motor 73 are all controlled by a controller 105.

Prior to the regular operation of the packaging machine 1, the heaterunit 50 and its support unit 60 are both in the non-work area while thepiston rod 71 a of the air cylinder 71 is fully pushed out. As theoperation of the packaging machine 1 is started, the controller 105 notonly controls the motor 73 to move the heater unit 50 and its supportunit 60 into the work area but also activates the first regulator 101and the first switch valve 103 such that the heater unit 50 and itssupport unit 60 will reach the sealing position. Additionally, thesecond regulator 102 and the second switching valve 104 are activated tocontrol the air pressure such that the heater belt 53 will contact themutually overlapping edge parts of the film and the compressive forcethereby on the film will be maintained at a specified level.

Should an abnormal condition be detected during this sealing operationsuch as the breaking of the film, the air cylinder 71 is operated suchthat heater unit 50 and its support unit 60 are moved to the retractedposition. When a time comes to exchange the chute 4 and the former 3,the heater unit 50 and its support unit 60 are moved not only to theretracted position within the work area but farther away into thenon-work area by means of the motor 73 such that a sufficiently largespace for the exchange can be secured.

As shown in FIG. 12, there is still another air passage 78 d providedthrough which air with the higher pressure prepared by the firstregulator 101 can be supplied into the air passage 78 c through thesecond switch valve 104. When the air passages 78 a and 78 b are bothclosed by the first switch valve and the air passage 78 c is opened bythe second switch valve such that air of the lower pressure is suppliedinto the second air supply chamber 71 d, the controller 105 is adaptedto operate the second switching valve 104 such that the air passages 78c and 78 d are connected through for a specified length of time andhence that air of the higher pressure is supplied into the air passage78 c. This is for the purpose of preventing a delay in the motion of thepiston member 71 b due to the static friction between the piston member71 b and the wall surface of the air cylinder 71 while air of the lowerpressure is being supplied and to thereby prevent the speed of motion ofthe heater unit 50 and its support unit 60 from becoming slowed down. Itnow goes without saying that the controller 105 of FIG. 12, too, mayalso be further adapted to preliminarily register a plurality of airpressure values such that automatic control or remote control operationscan be effected thereby.

1. A method of longitudinally sealing overlapping portions of atubularly formed bag-making material by means of a heater unit at asealing position, said method comprising the steps of: moving saidheater unit from a retracted position to said sealing position bysupplying air at a higher pressure to an air cylinder which serves tomove said heater unit reciprocatingly between said retracted positionand said sealing position; and thereafter supplying air at a lowerpressure which is lower than said higher pressure to said air cylinderand controlling said lower pressure to cause said heater unit tolongitudinally seal said bag-making material at a specified sealingpressure.
 2. The method of claim 1 wherein air at said lower and higherpressures is supplied from a single air source and by varying thepressure of air at said air source to said lower and higher pressuresand by using a switch means for selectively providing air at said lowerpressure or said higher pressure.
 3. The method of claim 1 wherein saidspecified sealing pressure is selected according to thickness of saidbag-making material.
 4. The method of claim 1 wherein said specifiedsealing pressure is selected according to a material characteristic ofsaid bag-making material.
 5. The method of claim 1 wherein said aircylinder has a first chamber and a second chamber, said first chamberserving to supply air for moving said heater unit to said retractedposition, said second chamber serving to supply air for moving saidheater unit to said sealing position.
 6. The method of claim 5comprising the step of supplying air of said lower pressure into saidsecond chamber of said air cylinder to control air pressure for sealingsaid bag-making material longitudinally at said specified sealingpressure.
 7. The method of claim 1 wherein air of said higher pressureis supplied to said air cylinder for a specified length of time whileair of said lower pressure is supplied to said air cylinder.